Two wire distributed control systems are used in many industrial settings for controlling and monitoring a plurality of devices. Such systems operate under particular protocols, of which one example is Fieldbus. This is the name of a family of industrial computer network protocols used for real-time distributed control, now standardized as IEC61158-2. A complex automated industrial system, for example a fuel refinery, usually needs an organized hierarchy of controller systems to function. In this hierarchy there is a Human Machine Interface (HMI) at the top, where an operator can monitor or operate the system. This is typically linked to a middle layer of programmable logic controllers (PLC) via a non time critical communications system (e.g. Ethernet). At the bottom of the control chain is the Fieldbus, which links the PLCs to the devices which actually do the work such as sensors, actuators, electric motors, console lights, switches, valves and contactors. The Fieldbus is usually a two wire combined power and data network, which provides both power and communications to the field devices.
In a typical combined two wire Fieldbus electrical power and communications circuit there is a power supply, an Intrinsic Safety barrier of some kind, a trunk section leading out into the field, and a number of device couplers with spurs connected thereto, on which the field instruments are mounted. The trunk and the spurs together form a “segment”. The Intrinsic Safety barrier divides the circuit into an Intrinsically Safe side and a non-Intrinsically Safe side. The power supply, the PLCs and other systems like physical layer diagnostic modules which measure physical layer attributes of the electrical circuit and the network hardware, and in part the physical software or protocol being used, are located in the non-Intrinsically Safe side of the circuit, usually in a control room. The trunk, the device couplers, the spurs and the field instruments are located in the Intrinsically Safe side, out in the field.
The various components of the system communicate with one another using the Fieldbus IEC61158-2 communications protocol, which is a Manchester encoding system. Data telegrams are transmitted either on dedicated communications circuits, or on the same electrical circuits as the power to drive the field instruments. The data telegrams serve to control and to monitor and diagnose the field instruments in use. However, it is also possible to use Fieldbus, or any other similar two-wire protocol, in ordinary non-Intrinsically Safe environments and without any Intrinsic Safety barrier.
The IEC61158-2 Fieldbus communication protocol, along with other similar data systems such as DSL, Ethernet, HART and so on, and those which will eventually replace IEC61158-2 Fieldbus, comprise a set of discrete physical layer limits within which the communications signals, and the hardware which hosts them, must operate. These physical layer limits include aspects of the signalling, including the data shape and timing rules. These limits and tolerances are very conservative, so the resulting systems are highly robust and not prone to failure. In addition, this conservative approach means that when a system is physically installed it usually operates correctly first time without the need for adjustment, even though in practice a system can operate somewhat differently to how it was initially designed.
The Fieldbus solution was introduced over a decade ago, and demand is now growing for a higher speed data rate to meet technology advancements and to increase the network efficiency. Therefore, Fieldbus instruments or devices will eventually become ‘legacy devices’ following the introduction and maturity of a field based higher speed communication standard, which may be an extension to the IEEE802.3 Ethernet standard, or it may be an extension to any other similar or future standard, not discounting an IEC61158-2 annex or supplement.
Because IEC61158-2 Fieldbus devices have a very large installed base, and because such devices are commodities that are often reused (or kept in place) when upgrading the process plant cabling infrastructure, it is desirable that any new platform or interface should be able to accommodate IEC61158-2 Fieldbus devices so that the transition cost, to the customer, can be kept low.
Customers may also want to keep IEC61158-2 Fieldbus devices uniform across a plant upgrade in order to keep the spares inventory as small as possible, yet benefit from a higher speed communication backbone, which can use the same existing cabling with minimal changes.
Furthermore, to establish a new high-speed backbone, allowing both new and legacy devices to optionally connect to the same interface switch port or spur port would be more acceptable to the customer. This would give the customer a greater selection of legacy and high speed instruments to choose from on the onset, when not all instrument variants are likely to be available with high speed communication. Upgrading to higher speed instruments when these become available at a later date will then be a simple process requiring minimal changes and/or expense.
It is therefore an aim of the invention to provide an interface circuit that can be used to connect to devices having different signalling protocols via a two wire combined power and data bus.